During the last decade experts have increasingly attended to reprocessing of used oils, which are also known as waste oils, under environmental safe conditions, whereby the term "contaminated" or "used" oils is to be considered as the more general definition.
Although a precise definition of the term "waste oil" is not available (see for example chapter 1 in K. Muller, "Reprocessing of waste oils", Erich Schmidt Verlag, Berlin 1982), the most general meaning embraces used lubricants and functional fluids, which cannot be used directly, and which are predominantly based on mineral oil, but may also be based on coal tar, plants, animals and synthesis. These materials may also be of solid or semi-solid consistency at normal temperature.
Unused lubricants and functional fluids contain numerous additives, like for example corrosion inhibitors, oxidation inhibitors, anti-foaming additives, detergents, dispersants, metal deactivators, colour stabilizers, viscosity index improvers, pour point reducing additives, wear control additives, emulsifiers etc.
By these additives, compounds of oxygen, sulfur, nitrogen, phosphorus, of metals like heavy metals, but also of other metals, halogens, silicons, numerous synthetic materials, and other compounds are present in lubricating oils and functional fluids.
During usage of fresh oil as lubricants and functional fluids, alterations of the additives take place by physical effects, like increased temperature and wear, but also by chemical processes, like for example oxidation. As a result the products of these transformations and changes are also present in used oils, like finely distributed metals, asphalthene materials, respectively materials of resin consistency and other secondary products formed by chemical changes of the additives.
Often contaminated oils of completely different origin are jointly collected. As a consequence the mixture may contain numerous solvents, water and solids. In these mixtures and in waste oils in general, the content of chlorinated compounds, in particular of polychlorodiphenyls, polychloroterphenyls, polychlorodiphenylmethans and other chlorine containing contaminations is of particular importance.
On account of these chlorocompounds, but also of metals and metal compounds, incineration of waste oils is very problematic, since the formation of toxic materials like dioxins or polychlorodibenzofurans and their emission into the environment cannot be excluded.
Regarding the very large quantities of waste oils and their importance as a raw material, incineration, which is also called energy recycling, i.e. recovery of heat energy by incineration, cannot be considered as an optimal way of handling, since materials, which are rich in hydrogen and which are therefore of great importance as fuels or feed materials for olefin plants, are only converted into water, CO.sub.2 and energy. As a consequence many processes have been developed for reprocessing of used oils into re-usable oils. Reprocessing of used oils is often called secondary refining.
The conventional process is the sulfuric acid process, whereby waste oils are treated with 96 weight percent sulfuric acid. In this process so-called acid resins are formed and deposited. In a subsequent stage neutralization is carried out and the oil treated in this way is finally distilled. In general this treatment is repeated. The remaining oils which still contain contamination, have a dark colour and bad odour. As a result they are difficult to re-use. In addition disposal of the acid resins is very prolematic.
An improvement is considered to be the sulfuric acid/clay-process, whereby after treatment of the oil with sulfuric acid it is contacted with clay and subsequently distilled under vacuum (R. Meixlsperger in W. Kumpf, K. Maas, H. Straub: Garbage and waste disposal, page 395, Nr. 4010).
extraction precedes the sulfuric acid treatment (R. Dutrian and D. V. Quang, Chemical Engineering 79 (1972), page 4). A more modern process comprising a refining step with hydrogen is the KTI-process (R. F. Westerduin in: Polytechnisch tijdschrift/process-techniek 34 (1979), page 5). This process is characterized by a first vacuum distillation, followed by a distillation under high vacuum. Finally a treatment with hydrogen at approximately 50 bars and 300.degree.-350.degree. C. is carried out.
Obviously, in spite of these processes, an urgent need continues to exist for solutions according to which in a technically simple way the large quantities of waste oils can be worked up under conditions which take into account environmental and economical requirements.
With regard to the direct reprocessing of waste oils together with crude oil in refineries, it is disclosed in K. Muller, "Reprocessing of waste oils", published by Erich Schmidt Verlag, Berlin 1982 on page 101: "Considering the original properties, the obvious possibility of adding waste oil to crude oil before processing of the latter, in order to increase the portion of lubricant oils, can be completely excluded owing to the typical contaminations present and the negative consequences caused by these contaminations with regard to catalysts and operating conditions applied in crude oil processing. For these reasons special secondary refining processes for the reprocessing of waste oils into high quality base lube oils are mandatory."